White Super Paint, Black Cats, and Colors We Don't Really See

"Big Blue Marble", NASA photo from Apollo 17
As we transition back into a semblance of normality from the pandemic lockdown, we found ourselves with a few mismatched pieces that didn't quite fit in previous posts, some updates from recent news articles, and a few new questions from readers that we wanted to deal with before we launch several new topic threads in this blog. So, we've presented some of them here in the format of one of our seasonal Q & A posts.

— J.D.

You mentioned a white paint that was somehow "super" in the Psychology of Color article. Can you tell us more about that?

Yes. Engineers at Purdue University have come up with an ultra-white paint. The process took nearly six years. The paint reflects approximately 98 % of sunlight, which has a major cooling effect. The idea is not new. People who live in very hot countries have used light-colored building materials or very white paints to cool their homes for centuries. The engineers used a compound of barium and sulfur. This isn't new either. The same compound is used in making photographic paper and in some cosmetics.

Painting roofs with this paint could cool houses and cars more efficiently than any type of air conditioning — and at far less cost. If used on a large scale on roofs and the tops of vehicles, perhaps even on ships, total energy use could be reduced markedly. Reduced energy use would reduce climate change over large portions of the planet.

Do you know that I had to look up more words in the dictionary to understand your article on The Psychology of Color than I have had to do for whole chapters of my high school textbooks? I've got pages and pages of notes!!!! I don't know if I will ever use some of them but I will know them if I see them again!!!! Whew! Should I thank you or what?

No need to thank me. You did all the work. I am impressed and extremely proud of you. I hope you are equally as proud of yourself. I had a group of girls in one of my classes who decided to learn a new word every day. They would get together at lunch to exchange lists. By the end of the year they had fat notebooks, too, but they never attempted to do as much in as short a time as you have done — and it was a collective effort. May I suggest one more addition to your notebook? Write a sentence for each word that is not the same as the sentence you may have found in your dictionary, but that is just as accurate. Then you will truly have command of that word. Congratulations!

You showed a picture of a black cat but didn't talk about it. How did it happen that people thought it was bad luck if a black cat crossed your path? Do you know?

That is a long and complicated story, but I will try to give you a condensed version. Cats, especially black ones, were revered as long ago as 3000 B.C. (BCE). They were often mummified after they died and placed in tombs. They kept the mice and rats out of food storage and were good companions. To kill one was a death sentence. The Japanese also held cats in high esteem.

Then came the Middle Ages in Europe (500 to 1500 AD). a time when literacy wasn't widespread and superstitions flourished. The Spanish appear to be the first to associate black cats with witches. The Irish, Welsh, and Scots didn't seem to take the idea seriously, but the English and Germanic tribes did. It is an English story that seems to have done the damage. It goes like this: a man and his son were walking along a path one evening when a black cat appeared on the same path. They threw rocks at it to drive it away and struck it hard on one leg. It ran into the house of a lone woman whom some people thought practiced witchcraft. The next day the man and his son saw the woman, who was limping. These two "geniuses" then told everybody that the limp was from a rock they threw at her the night before when she was in the body of a black cat. The story spread like wildfire and became the cause of the idea that it was bad luck to have a black cat cross your path. During the long pandemic of Bubonic Plague, people killed thousands of black cats. Now that caused REALLY bad luck because the plague was carried by mice and rats, which those cats could have killed and kept the plague from spreading! The Europeans brought their superstitions to America, causing such disasters as the Salem Witch Trials, in which 200 people were accused of witchcraft — and 19 of them were hanged.

There is a story going around that it is difficult to get people to adopt black cats from shelters, but the statistics don't support it. Of all the many cats my family has lived with, more were black than any other color. A word of caution, though. Black cats are still injured and killed at Halloween, so it is a good idea to keep yours safely indoors at that time. Enjoy your black cats!

Some time ago, in the very first issue of this series on Color, we briefly discussed structural color and gave an example or two. We discussed the fact that some of the colors we think we see are not really there. They are illusions produced by complex layers of structures. Since one of the colors we discussed in Part I of The Psychology of Color was Blue, this might be a good time to warn you that most of the things that we think we see as blue are products of structural color. The sunny blue sky we see is one of them. So is the deep blue of the ocean. And the feathers of the bluebird are not really blue. Which brings up several questions that I will try to answer briefly. (If you are really interested, there are many discussions of these topics on line.)

Why does the sky look blue to us in the daytime when outer space looks black?

The earth has a thick cover of atmosphere that has been forming for millions of years. The light from the sun hits molecules of various gases in that atmosphere and they fly off in all directions. The blue color of the sky is the result of this scattering. Because the Earth rotates, the light changes as the planet turns away from the Sun each day, leaving the sky black at night. If you were on the moon, which doesn't have an atmosphere, the sky would look black both day and night. Pictures taken by early astronauts show that very clearly.

Why does the ocean look blue when water doesn't really have much color?

According to NOAA (National Oceanic and Aimospheric Administration), it gets its color because of the way it absorbs sunlight. It strongly absorbs long wavelength rays of the color spectrum as well as short wavelength ones, even into the ultraviolet range. The remaining light that we see reflected from the ocean is mainly made of blue wavelengths. Objects floating just under the surface, such as algae and mats of seaweed, often give the ocean a green hue. If you dive below the surface and continue down the water column, the water becomes darker and darker until all color disappears into the darkness.

Why does a feather look blue if it doesn't have any blue pigment in it?

Hyacinth macaw (public domain)

If you hold a feather of any bright color that does not have any pigment in it up to a light, it loses its color and becomes more or less white (sometimes a slightly dirty-looking white). It becomes obvious that the color you originally saw on the feather was structural color. Move the feather back and look at it in normal light, and its color reappears. The structure of the feather itself determines what color it appears without pigment. Feathers that appear blue are covered with tiny air pockets that reflect short blue wavelengths. These pockets are actually filled with liquid when the feathers form, but the liquid is quickly replaced by air. If the pockets are arranged in a very regular way, the feathers will be a consistent color of blue like the wings of a macaw. If the arrangements are not quite regular, the feathers may show several different blues. (Think of a blue jay's feathers.)

There are very few animals that are truly blue. However, if you watch for our Frogs for Fun issue this summer, we will introduce you to a true blue one.

Do something colorful!